_P. platessa_, the Plaice: scales small, mostly without spinules, reduced and not imbricated, imbedded in the skin; bony knobs on the head behind the eyes, red spots on the upper side. _P. flesus_, the Flounder: no ordinary scales; rough tuberoles along the bases of the marginal fins and along the lateral line; these are modified and enlarged scales; elsewhere scales of any kind are absent. In these two species the lateral line is nearly straight, having only a slignt curve above the pectoral fin. _P. limanda_, the Dab: scales uniform all over the body, with spinules on the projecting edges, making the skin rough; lateral line with a semicircular curve above the pectoral fin. _P. microcephalus,_ the Lemon-dab: scales small, smooth, and imbedded; skin slimy, head and mouth very small, colour yellowish brown with large round darker marks. _P. cynoglossus,_ the Witch or Pole-dab: head and mouth smaller than in the Plaice, eyes rather larger; scales all alike and uniformly distributed, slightly spinulate on upper side, smooth on the lower; blister-like cavities beneath the skin of the head on the lower side. With regard to the generic characters, it is difficult to give any reason why the mouth should be at the end of the head instead of behind the apex of the snout as in the genus _Solea,_ but, as we have seen already, the small size of the mouth and the greater development of teeth on the lower side are adapted to the food and mode of feeding. It is impossible to say why one genus of Flat-fishes should have the right side uppermost and others, _e.g._ Sole and Turbot, the left; it would almost seem to have been a matter of chance at the commencement of the evolution: reversed specimens occur as variations in most of the species.